1. Field of the Invention
This application pertains to the field of pulse power drivers, particularly for use in high voltage, high frequency applications. The invention is particularly useful for ultrasound medical imaging systems to provide a controlled amplitude, power burst to a piezoelectric transducer element.
2. Description of the Prior Art
A typical ultrasound imaging system used for medical imaging includes at least one ultrasound transducer, usually in the form of an array of transducer elements. The transducer is usually connected to transmitter stage circuitry, receiver stage circuitry and whatever devices may be necessary to effect mechanical scanning of the transducer. The transmitter stage comprises a generator of electrical excitation signals which are sent to the transducer. The transducer converts these signals into periodic pulse trains of ultrasound mechanical energy.
Transducer arrays used in medical imaging systems come in many forms, such as linear arrays, phased arrays, annular arrays and collimated image arrays. A transducer array usually consists of a plurality of transducer elements disposed on a surface in some designed arrangement such as a row, matrix or other geometric pattern. The individual elements or groups of elements are actuated or pulsed in sequence to transmit a beam of ultrasound energy at a field of interest. Ultrasound echoes are returned from the target and may be received on the same transducer elements. The pulse echo data received is then interpreted and displayed to produce an image.
A transducer may be apodized by shaping the distribution of energy applied across the transducer to a desired aperture function. For a single disc, piezoelectric transducer, this has been accomplished by shaping the applied electric field through use of different electrode geometries on opposite sides of the disc as described, for example, in Martin and Breazeale "A Simple Way to Eliminate Diffraction Lobes Emitted by Ultrasonic Transducers", J. Acoust. Soc. Am. 49 No. 5 (1971) 1668, 1669 or by applying different levels of electrical excitation to adjacent transducer elements in an array. However the method of Martin and Breazeale is limited to a number of simple fixed aperture functions and the use of separate surface electrodes requires complex transducer geometries and/or switching circuits.
Precise control of the potential fed to each transducer element of an array is essential for apodization in the transmit stage of a pulse echo imaging system. Ultrasound transmitters in present use are of the kind known as "voltage mode drivers". Typically an ultrasound transducer element is connected via a solid state switch to a fixed, high voltage supply. During transmission, the switch is closed for a short time, or in a repetitive manner, to produce a series of pulses across the transducer. The amplitude of these pulses is related to the voltage level of the supply and can only be controlled by changing the power supply potential. For an application that employs a linear array transducer of more than 100 elements, especially a shifting array, the precise control of this low impedance potential would necessitate a tremendous increase in complexity since each transducer element driver would require a separate controllable (voltage) supply.
The present invention describes a circuit which solves the problem of providing a variable voltage level to each transducer element in an array.